1. Technical Field
This invention relates in general to offshore drilling and production equipment, and in particular to a subsea well system for monitoring the pressure in a non-producing string of casing through the completion system.
2. Description of Related Art
A subsea well that is capable of producing oil or gas will have a conductor housing secured to a string of conductor pipe which extends some short depth into the well. A wellhead housing lands in the conductor housing. The wellhead housing is secured to an outer or first string of casing, which extends through the conductor to a deeper depth into the well. Depending on the particular conditions of the geological strata above the target zone (typically, either an oil or gas producing zone or a fluid injection zone), one or more additional casing strings will extend through the outer string of casing to increasing depths in the well until the well is cased to the final depth. Each string of casing is supported at the upper end by a casing hanger. The casing hanger lands in and is supported by the wellhead.
In some shallow wells and in some fluid injection wells, only one string of casing is set within the outer casing. Where only one string of casing is set within the outer casing, only one casing hanger, the production casing hanger, is landed in the wellhead housing.
The more typical case is where multiple strings of casing are suspended within the wellhead housing to achieve the structural support for the well to the depth of the target zone. Where multiple strings of casing hangers are landed in the wellhead housing, each casing hanger is above the previous one in the wellhead housing. Between each casing hanger and the wellhead housing, a casing hanger packoff is set to isolate each annular space between strings of casing. The last string of casing extends into the well to the final depth, this being the production casing. The strings of casing between the outer casing and the production casing are intermediate casing strings.
When drilling and running strings of casing in the well, it is critical that the operator maintains pressure control of the well. This is accomplished by establishing a column of fluid with predetermined fluid density inside the well. During drilling operations, this fluid is circulated down into the well through the inside of the drillstring out the bottom of the drillstring and back to the surface. This column of density-controlled fluid balances the downhole pressure in the well. When setting casing, the casing is run into the pressure balanced well. A blowout preventer system is employed during drilling and running strings of casing in the well as a further safety system to insure that the operator maintains pressure control of the well. The blowout preventer system is located above the wellhead housing by running it on drilling riser to the wellhead housing.
When each string of casing is suspended in the wellhead housing, a cement slurry is flowed through the inside of the casing, out of the bottom of the casing, and back up the outside of the casing to a predetermined point. An open fluid communication passage in the casing hanger leading from the casing annulus to the casing interior would adversely affect the flow path of the cement slurry. This could also cause well pressure control problems for the operator under certain conditions.
In a subsea well capable of producing oil or gas, the production fluids flow through perforations made in the production casing at the producing zone. A string of tubing extends to the producing zone within the production casing to provide a pressure controlled conduit through which the well fluids are produced. At some point above the producing zone, a packer seals the space between the production casing and the tubing to ensure that the well fluids flow through the tubing to the surface. The tubing is supported by a tubing hanger assembly that lands and locks above the production casing hanger, either in the wellhead housing, in a tubing hanger spool, or in a horizontal or spool tree, as described below.
Subsea wells capable of producing oil or gas can be completed with various arrangements of the production control valves in an assembly generally known as a tree. For wells completed with a conventional tree, the tubing hanger assembly lands in the wellhead housing above the production casing hanger. Alternatively, the tubing hanger assembly lands in a tubing hanger spool that is itself landed and locked to the wellhead housing. For wells completed with a horizontal or spool tree, the horizontal tree lands and seals on the wellhead housing. A tubing hanger assembly lands and seals in the horizontal tree. The tubing hanger assembly in conventional trees has a flow passage for communication with the annulus surrounding the tubing. A tubing annulus bypass extends around the tubing hanger in horizontal trees. These passages allow for communication between the interior of the production casing and the interior of the tubing. Virtually all producing wells are capable of monitoring pressure in the annulus flow passage between the interior of the production casing and the interior of the tubing.
A sealed annulus locates between the production casing and the next larger string of casing. Normally there should be no pressure in the annulus between the production casing and the next larger string of casing because the annular space between the production casing and the next larger string of casing is ordinarily cemented at its lower end and sealed with a packoff at the production casing hanger end. If pressure within this annulus increases, it would indicate that a leak exists in one of the strings of casing. The leak could be from several places. Regardless of where the leak is coming from, pressure build up in the annulus between the production casing and the next larger string of casing could collapse a portion of the production casing, compromising the structural and pressure integrity of the well. For this reason, operators monitor the pressure in the annulus between the production casing and the next larger string of casing in land-based or above water wells. Monitoring production casing annulus pressure in a subsea well is more difficult because of lack of access to the wellhead housing below the production casing hanger packoff. Patents exist that show different methods for monitoring the annulus pressure between the production casing and the next larger casing in subsea wells.